The invention relates to a method and apparatus for spinning a multifilament yarn from a thermoplastic material, and of the general type disclosed in EP 0 682 720 and corresponding U.S. Pat. No. 5,976,431.
In the spinning by the known method and apparatus, an air stream assists the freshly extruded filaments in their advance. With that, it is accomplished that the solidification zone of the filaments moves away from the spinneret. This again leads to a delayed crystallization, which has a favorable effect on the physical properties of the yarn. For example, in the production of a POY yarn, it was possible to increase the withdrawal speed and, thus, the draw ratio, without changing the elongation values for the yarn, which are necessary for its further processing.
To this end, the known apparatus comprises downstream the spinneret, a cooling device, which includes an upper cooling shaft and a lower cooling shaft connected to the upper cooling shaft. At its outlet end, the lower cooling shaft connects to a cooling stream generator, which generates a vacuum in the lower cooling shaft. The upper cooling shaft is made gas permeable, so that the vacuum prevailing in the lower cooling shaft causes an air stream to flow into the upper cooling shaft and to advance in the direction of the lower cooling shaft. In so doing, a coolant stream is generated, which has a flow velocity substantially equal to the advancing speed of the filaments. This influences friction between the filaments and the adjacent air layer such that crystallization starts with a delay, and the filaments solidify in a solidification zone within the lower cooling shaft.
However, it has shown that in the spinning of fine filament deniers, for example 1 dtex/f or less, crystallization in the filaments has progressed, after a precooling in a cooling zone formed by the upper cooling shaft, to such an extent that the subsequent assistance in the continuing advance no longer shows a significant influence for delaying crystallization.
U.S. Pat. No. 4,277,430 discloses a method and apparatus, wherein the filaments are cooled in the cooling zone downstream of the spinneret by directing thereto a transverse air flow. Subjacent the cooling zone is a second cooling shaft, which receives in its inlet area an air/water mixture as a misty cooling stream. For cooling the yarn, the misty cooling stream is caused to flow by means of suction in the direction of the advancing yarn to the end of the cooling zone. In this process, the addition of liquid realizes a yet greater cooling effect on the filaments, so that the onset of crystallization is not delayed, but accelerated.
It is an object of the present invention to further develop a method of the initially described kind as well as an apparatus for carrying out the method in such a manner that it becomes possible to produce yarns with low, medium, or high deniers at higher production speeds and with uniform physical properties.
The invention is based on the knowledge that from their emergence from the spinneret to their solidification and formation of the yarn, crystallization of the filaments is determined by two mutually influencing effects. It is known that during the cooling of a polymer melt, the melt solidifies at a certain temperature. This process is dependent solely on the temperature, and herein named thermal crystallization. In the spinning of yarns, a filament bundle is withdrawn from the spinnerets. In this process, the yarn is subjected to withdrawal forces, which effect a tension-induced crystallization in the filaments. Thus, in the spinning of yarns, thermal crystallization and tension-induced crystallization are superposed, and jointly lead to the solidification of the filaments. To influence tension-induced crystallization, the filament bundle is guided, prior to its solidification, into a tension zone, in which the yarn friction and, thus, the yarn tension acting upon the yarn are changed.
Thus, the invention makes available a method and an apparatus, which make it possible to influence tension induced crystallization under substantially unchanged conditions. To this end, the cooling of the filaments, after their emergence from the spinneret, is adjusted within the cooling zone such that the location of the solidification zone of the filaments is kept within the tension zone in a predetermined desired range thereof. Thus, solidification of the filaments in the tension zone in the lower cooling shaft always occurs essentially in the same place, so that a uniform treatment of the filaments is ensured for influencing tension induced crystallization. To influence thermal crystallization, it is necessary that the cooling effects, which the coolant exerts in the cooling zone, be made variable. However, in this connection, it is necessary that before their entry into the tension zone, the filaments already have a certain stability, in particular in their outer edge layers, for purposes of withstanding undamaged the coolant stream, which is generated in the tension zone for treating the yarn tension. A particularly advantageous variant for controlling the cooling is provided by a further development of the invention, wherein the coolant is tempered before entering the cooling zone. In this instance, the temperature of the coolant may be increased to a value preferably in a range from 20xc2x0 C. to 300xc2x0 C. To spin, for example, a yarn with a relatively low filament denier, the coolant is preheated to a higher temperature by a heating device, which is used as a means. This influences thermal crystallization in such a manner that the filament bundles are not solidified before they enter the tension zone. Thus, an advantageous tension treatment is possible by a coolant stream directed parallel to the filaments. This stream causes the filaments to solidify in the desired range of the tension zone. In the case that it is intended to spin a yarn of a high denier, the coolant will be adjusted to a lower temperature, so that before entering the tension zone, thermal crystallization has developed so far that the filaments exhibit adequate stability when they are attacked by the coolant stream.
To adjust cooling in the cooling zone, a further advantageous improvement of the invention proposes to change the volume flow of the coolant. The means used to this end is a blower, which can be used to control the volume flow that is blown into the cooling zone.
At this point, it should be noted that basically all known means for influencing the cooling effect in the cooling zone are suitable for using the method of the present invention for spinning a yarn. The herein described means are especially suited for the instance, when a cooling air is used as coolant. For example, when a vaporous coolant is used, it would be possible to influence the cooling effect solely by the state of the vapor. Likewise, it is possible to use means in the form of devices for influencing cooling in the cooling zone, such as, for example, movable sheet elements, which influence the entry of the coolant into the cooling zone.
To ensure a great uniformity in the spinning of the filaments, a preferred further development of the invention provides that the coolant stream is accelerated to the flow velocity necessary for treating the tension of the filament bundle, only in an acceleration zone within the tension zone. In so doing, the coolant stream is accelerated at least to a flow velocity, which equals the speed of the advancing filaments, so that the filaments are not decelerated in their continuing movement. Thus, for reaching an optimal tension induced crystallization, the desired zones for solidifying the filaments extend within or directly downstream of the acceleration zone of the coolant.
The coolant stream in the tension zone may be generated from the coolant leaving the cooling zone and from a coolant supplied in the inlet area of the tension zone downstream of the cooling zone. This construction permits the tension induced crystallization to be adjustable within a wide range. The additionally supplied coolant further permits an influencing of the cooling of the filament bundle in the tension zone. In particular, in the spinning of yarns with high deniers, the supply of an additional coolant makes it possible to achieve a desired minimum cooling at the outlet end of the tension zone when the yarn is combined.
The method of the present invention is independent of whether the coolant stream is generated in the tension zone by a suction effect or by a blowing action. The variant of the method, wherein a suction flow prevails in the tension zone, has the advantage that thermal crystallization in the cooling zone and tension induced crystallization in the tension zone can be influenced substantially independently of each other.
To generate a coolant stream by a blowing action, it is possible to blow the coolant into the cooling zone and to guide it correspondingly into the tension zone, or to blow the coolant supplied downstream of the cooling zone directly into the tension zone.
To obtain an effect of the coolant stream, which is as uniform as possible on each filament of the filament bundle, the tension zone may be formed by a cooling duct through which the filaments advance, and which has on its inlet end a narrowed cross section which operates as an acceleration zone for the air entering the duct.
Based on its flexibility, the method of the present invention is especially suited for spinning yarns of polyester, polyamide, or polypropylene. An aftertreatment of the yarn, which is suitable after spinning, makes its possible to use the method for producing, for example, a fully drawn yarn (FDY), a partially oriented yarn (POY), or a highly oriented yarn (HOY)
The method of the present invention can be carried out very advantageously by an apparatus, wherein the cooling device comprises an upper cooling shaft and a lower cooling shaft. The upper cooling shaft extends directly downstream of the spinneret, and forms a cooling zone, in which thermal crystallization is influenced by a coolant introduced into the cooling shaft. The lower cooling shaft connects to the upper cooling shaft, and forms the tension zone. To generate a coolant stream flowing parallel to the yarn, the cooling device includes a cooling stream generator. This cooling stream generator is used to generate a coolant stream with a predetermined flow velocity. According to the invention, the apparatus for carrying out the method comprises a means for adjusting the cooling of the filaments in the upper cooling shaft. This means permits influencing the cooling of the filaments in such a manner that the filaments solidify only in a predetermined desired range of the lower cooling shaft. Thus, the apparatus of the present invention is suitable for changing the location of the solidification zone of the filaments along the spin line, in particular in the region of the lower cooling shaft. It is possible to use as means both such devices, which are operative on the cooling device and such devices, which directly act upon the coolant.
Advantageously, with the use of cooling air, the means is designed and constructed as heating device, which tempers the cooling air entering the lower cooling shaft. In this instance, the heating device is operated via a controller with corresponding, predetermined control values.
To generate in the lower cooling shaft an as uniform coolant stream as possible, it is especially advantageous to form an acceleration zone in the cooling shaft by means of a narrowed cross section. A coolant entering the lower cooling shaft is thus accelerated to a flow velocity, which essentially depends on the pressure difference prevailing between the inlet side and the interior of the lower cooling shaft.
To generate the pressure difference for developing a coolant stream in the lower cooling shaft, it is possible to utilize as the cooling stream generator both a blower, which blows the coolant into the lower cooling shaft, and a source of vacuum, which connects to the lower cooling shaft on the outlet side thereof, and sucks the coolant into the lower cooling shaft.
To produce qualitatively superior yarns, the lower cooling shaft may be formed by a tube, through which the filament bundle advances. The inlet end mounts a condenser and the outlet end a diffuser. The condenser generates a uniform coolant stream, which surrounds the filament bundle. The diffuser produces a slow decrease of the flow velocity of the coolant stream, so that the filament bundle advances through the lower cooling shaft substantially with little turbulence.
To improve the smooth run of the filament bundle and to avoid stronger turbulence in the cooling shaft, a very advantageous further development of the apparatus provides for a second condenser between the upper and the lower cooling shafts. This second condenser ensures a substantially turbulencefree transition of the coolant from the upper cooling shaft to the lower cooling shaft. In this instance, the acceleration zone, which is characterized by the narrowest flow cross section, may be formed both in the first or in the second condenser. To increase the cooling effect, in particular in the case of coarse yarn deniers, it will be advantageous to introduce an additional coolant into the tension zone between the two condensers.